1. Field of the Invention
The present invention relates to a pH sensor for measuring the pH of a liquid to be measured based on outputs of a reference electrode and a working electrode.
The present invention also relates to ion sensors, particularly a pH sensor among the ion sensors, for measuring an ion concentration of a liquid to be measured based on an output of a reference electrode and an output of a working electrode.
Priority is claimed on Japanese Patent Application No. 2011-026194, filed Feb. 9, 2011, and Japanese Patent Application No. 2011-031102, filed Feb. 16, 2011, the contents of which are incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
A glass electrode type pH sensor is disclosed in Japanese Unexamined Patent Application, First Publication No. 1997(H09)-005290, in which the pH of a liquid to be measured is measured by detecting a potential difference between the inside and outside of a glass film using a glass electrode that functions as a working electrode. In the pH sensor disclosed in Japanese Unexamined Patent Application, First Publication No. 1997(H09)-005290, a reference electrode is disposed in an internal liquid, such as a potassium chloride solution (KCl solution), that is filled in a holder. Further, a ceramic liquid junction is installed in the holder. When the pH is measured, the glass film and the ceramic liquid junction are dipped into the liquid to be measured. In this case, an internal electrode of a working electrode is electrically connected to an inner wall of the glass film via the internal liquid. On the other hand, the reference electrode is electrically connected to an outer wall of the glass film via the internal liquid, the ceramic liquid junction, and the liquid to be measured. Accordingly, the pH of the liquid to be measured can be measured by detecting a potential difference between the reference electrode and the internal electrode of the working electrode.
In the glass electrode type pH sensor disclosed in Japanese Unexamined Patent Application, First Publication No. 1997(H09)-005290 or Japanese Unexamined Patent Application, First Publication No. 2007-078373, problems occur such as contamination of the liquid to be measured caused by leakage of the internal liquid, which is contained inside a layer of the holder in which the reference electrode is held, into the liquid to be measured, a change in state caused by moisture vaporization of the internal liquid, crystallization of the internal liquid, and the like.
Further, an ion sensor is disclosed in Japanese Unexamined Patent Application, First Publication No. 2009-236687, which includes first and second ion sensitive field effect transistors (ISFETs), and provides a difference in sensitivity (Nernst response) to measuring-target ions of the two ISFETs.
In the ion sensor disclosed in Japanese Unexamined Patent Application, First Publication No. 2009-236687, a self-assembled monolayer (SAM) is used for ion sensitive and insensitive parts of the ISFET. As such, the SAM has a problem with physical and chemical instability in a high-temperature, high-pressure state of a chemical synthesis plant, or in a strong acid or alkaline process of a semiconductor fabricating process, and thus is difficult to use. Further, in a bioprocess of handling bio-related materials such as proteins, the protein is adsorbed to the SAM, making accurate measurement difficult.